| structures, arrays of pointers and assignment [message #53752] |
Wed, 02 May 2007 06:22 |
Ingo von Borstel
Messages: 54
Registered: September 2006
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Member |
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Hi there,
with one of my routines I found an issue that I cannot explain myself.
Maybe someone might put me on the right track as to where things go
wrong. Probably it's an issue with passing variables by reference and by
value, but I don't quite understand the difference in the two things I
do. Assume the two variables:
IDL> help, sep213.trackvalues
<Expression> STRUCT = -> TRACKVALUES Array[1]
IDL> help, myvalues
MYVALUES STRUCT = -> TRACKVALUES Array[1]
where trackvalues is a struct:
trackvalues = {trackvalues, $
value:PTRARR(4), $
used: BYTARR(4), $
description:STRARR(4), $
timebase:'' $
}
and sep213 is a struct, too, that contains trackvalues as one item.
I have written a routine to manipulate the contents of trackvalues:
PRO do_something, in_trackvalues
; create just another struct, details of it unimportant
newentry = {values:dblarr(300), used:bytarr(300), descr:''}
; create a pointer to this struct
ptrnew = ptr_new(newentry)
; ensure to get a valid index in the array in_trackvalues.value so that
I can assign the above created pointer to this index
wherefree = <some code to ensure I get a valid index so that the next
line is a valid assignment>
in_trackvalues.value[wherefree] = ptrnew
END
Now I call this routine from the IDL prompt:
IDL>do_something, sep213.trackvalues
or
IDL>do_something, myvalues
and get different results:
IDL> print, sep213.trackvalues.value[0:3]
<PtrHeapVar77176><PtrHeapVar77177><PtrHeapVar77178><NullPointer >
IDL> print, myvalues.value[0:3]
<PtrHeapVar77176><PtrHeapVar77177><PtrHeapVar77178><PtrHeapVar77973 >
The latter case is what I expect as I create an additional, valid entry
in the ptrarr array. Why is this entry forgotten in the first case? I
checked whether the routine works in the first case. It does up to the
last line exactly the same, only there is no modified data returned to
the calling routine.
Any clue as to why? My guess it's to do with passing elements of a
struct prohibits its modification by a subroutine. If so, is there a way
to circumvent this behaviour?
Best regards,
Ingo
--
Ingo von Borstel <newsgroups@planetmaker.de>
Public Key: http://www.planetmaker.de/ingo.asc
If you need an urgent reply, replace newsgroups by vgap.
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